The present invention relates to a method for preparing transparent, electrically conducting indium oxide (In.sub.2 O.sub.3) films on temperature sensitive substrates by high-frequency cathode sputtering of indium-tin alloy targets in a vacuum, using a reaction gas.
All methods and substances used to date for the preparation of such electrically conducting films, such as tin-doped indium oxide (In.sub.2 O.sub.3 : Sn), antimony-doped tin oxide (SnO.sub.2 : Sb) and cadmium stannate (Cd.sub.2 SnO.sub.4) require, either during the preparation or in subsequent processing, the application of a temperature treatment at temperatures between about 200.degree. C. and 700.degree. C. in order to insure the attainment of optimum electrical and optical properties of the films. It does not matter whether an oxygen deficit produced in the preparation through the temperature treatment is to be satisfied or whether an oxygen excess is to be driven out (J. Vac. Sci. Technol. 15 (4), July/Aug. 1978, pages 1565 to 1567).
For certain applications, for example, if film substrates which are not thermally stable, such as plastic foils or optical fibers, are used or if the application of the film is to be made on top of thermally unstable layers already deposited in a previous operation, heating during the deposition process or a subsequent temperature treatment is undesirable.
To date, for example, very thin, partially transparent metal films of gold have been used as substrates. However, the light absorption of gold is very high (about 50% at a thickness of 10 nm) and the low mechanical hardness and the poor adhesion of these layers are also disadvantageous.
Additionally, in the attempts thus far to precipitate oxide semiconductor layers on cold substrates, either optimum optical transparency or optimum conductivity, depending on the intended application, had to be sacrificed (Appl. Phys. 16, pgs. 239 to 246 (1978); Japanese Journal of Applied Physics Vol. 17, No. 7, July 1938, pgs. 1191 to 1196).
While it has been possible to deposit conducting transparent films of In.sub.2 O.sub.3 on water-cooled polyester foils or glass by high-frequency ion plating in argon-oxygen mixtures, the values for the resistivity obtained, with .rho.=8.times.10.sup.-3 ohm.cm, are still about 40-times higher than those which are known for the same layers with thermal post-treatment.